The invention relates to an image reader that causes an image reading optical system including a color reading sensor to scan a document region.
An exemplary image reader including a color reading sensor is shown in FIG. 4. In FIG. 4, reference numeral 1 designates an image reader unit; 1a, a housing thereof; 2, a document; 3, a platen glass; 4, a fluorescent lamp; 5, a reflecting mirror; 6, a rod lens array; 7, a filter; 8, a CCD (charge-coupled device) color line sensor; 8a, a connecting member; 9, a circuit board; 10, a lamp heater; 11, a control signal flexible cable; 12, an illumination power supply flexible cable; 13, a radiation plate; 13a, a projecting portion; 14, an electromagnetic shield punching drive; 15, an IC (integrated circuit) chip; and 16, a reference white plate.
FIG. 5 is an electric block diagram of the image reader. An optical output from a document read by the sensor 8 is converted into a digital signal by an A/D converter circuit 17, and the digital signal is then applied to an image output terminal 19 through an image processor circuit 18.
If the fluorescent lamp 4 is used as a light source in such an image reader, an image reader having the characteristics that the warmup time is short and that the lamp heater power is set to a low value because the optical output is temperature-dependent and must start copying even if the lamp tube wall temperature is still low and has not yet reached an optimal temperature. If the image reader starts a continuous copying operation at this point, the lamp tube wall temperature rises due to heating by itself with the lamp being turned on, thus causing fluctuations of the quantity of light.
A lamp tube wall temperature rising characteristic at 10.degree. C. is shown in FIG. 6. It is assumed here that the lamp heater power cannot be increased and that the warmup time must be one minute. FIG. 6 shows an operation sequence of an image reader, a solid line indicating a nominal condition and broken lines including an upper and lower limits of an acceptable tolerance. This image reader exhibits a characteristic that the heater temperature is risen to 37.degree. C. under the nominal condition; that the heater is turned off upon turning the lamp on at 37.degree. C.; and an AGC/AOC operation is started upon turning the lamp off after 55 seconds. Further, the control temperature during standby is 50.degree. C.
With the characteristic shown in FIG. 6, to satisfy the one-minute warmup time, the lamp must be turned on at about 19.degree. C. and a copying operation is then started under the condition of the nominal heater power (the solid line). As a result, the fluctuations in the quantity of light, such as shown in FIG. 7, are observed, i.e., the quantity of light changes noticeably while making initial 10 copies or performing 40 scanning operations. The reason is that the optical output of the fluorescent lamp depends on the temperature and has the characteristic shown in FIG. 8.
In FIG. 8, the optical output of the fluorescent lamp peaks with the tube wall being at 45.degree. to 50.degree. C., and decreases with decreasing temperature. Assuming that its peak output is 100%, then the optical output at 19.degree. C. becomes about 40% of the peak value. If the copying operation is performed continuously under this condition, the tube wall temperature of the fluorescent lamp changes drastically, thus causing the conspicuous fluctuations in the quantity of light, as shown in FIG. 7.
In the case of the image reader, a sensor output of the CCD color line sensor 8 with respect to the reference white plate 16 is monitored, and an analog gain must be set before A/D conversion at the A/D converter circuit 17 (see FIG. 5) that comes after sensing by the sensor. This operation is called "AGC," and the normal operation sequence is that AGC is effected before the image reader becomes ready to start copying (READY).
Let us take the case shown in FIG. 7 as an example and assume that AGC is effected after the lamp has been turned on during warmup. This timing corresponds to the zero copy state in FIG. 7. Assuming that the quantity of light is 100% at this timing, as the continuous copying operation is proceeded, the quantity of light increases to a level of as much as 250%. This makes AGC useless and the increase in the quantity of light causes the AGC set value to overflow (either the output of the amplifier or the output of the A/D converter becomes saturated).
Consequently, a normal reproduction characteristic such as D.sub.OUT shown in FIG. 9 cannot be obtained, producing a very low-quality image whose low-density portion is not well reproduced as shown in FIG. 10.